Silicon has shown dominance over other materials over the years but, because of its indirect band gap nature, it was believed that it cannot find use in photonic devices. However, very recently there were some successes to make light emitting diodes and Laser using nano-silicon. References in this connection may be made to M. A. Green, J Zhao, A Wang, P J Reece, and M GAL in Nature 412, 805 (2001) and also L. Pavesi L Dal Negro, G Mazzoleni, G Franzo and F Priola in Nature 408, 440 (2000).
The properties of silicon undergo significant changes when the crystallite size reaches the range of a few nanometers. The nanometer-scale size of these crystals leads to novel electronic and optical properties associated with quantum confinement and single-electron charging effects. Because of their technological applications such nanocrystalline silicon are receiving increasing attention. It appears now that nanocrystalline silicon could provide the alternate to other direct band gap semiconductors, e.g., gallium arsenide and indium phosphide that are good at emitting light but are more expensive than silicon. The properties of nanocrystalline silicon have also been exploited for the fabrication of various other devices such as single-electron transistors and memories for which references may be made to K. Yano, T. Ishii, T. Hashimoto, T. Kobauashi, F. Murai, and K. Seki in Appl. Phys. Lett. 67, 828 (1995) and Y. T. Tan, T. Kamiya, Z. A. K. Durrani, and H. Ahmed, in J. Appl. Phys. 94, 633 (2003); electron emitters for which reference may be made to K. Nishiguchi, X. Zhao, and S. Oda in J. Appl. Phys. 92, 2748 (2002); field effect electroluminescence for which reference may be made to K. Nishiguchi, X. Zhao, and S. Oda in J. Appl. Phys. 2, 2748 (2002).
In recent years, there has been great interest in silicon nanocrystals prepared by various growth techniques. References may be made to K. Yano, T. Ishii, T. Hashimoto, T. Kobauashi, F. Murai, and K. Seki in Appl. Phys. Lett. 67, 828 (1995); G. F. Grom, D. J. Lockwood, J. P. McCaffery, N. J. Labbe, P. M. Fauchet, B. White, Jr., J. Diener, D. Kovalev, F. Koch, and L. Tsybeskov in Nature 407, 358 (2000); S. Oda and M. Otobe in Mater. Res. Soc. Symp. Proc. 358, 721 (1995); T. Kamiya, K. Nakahata, Y. T. Tan, Z. A. K. Durrani, and I. Shimuzu in J. Appl. Phys. 89, 6265 (2001). It has been shown that porous silicon prepared by anodic etching of silicon wafers in an HF/ethonal/water solution, with a crystallite size of few nanometers shows an efficient photoluminescence. References may be made to L. T. Canham in Appl. Phys. Lett. 57, 1046 (1990); V. Lehmann and U. Gosels in Appl. Phys. Lett. 58, 856 (1991). However, there is disadvantage of porous silicon like its brittleness, high porosity and a relatively poor control of the crystallite size, which are major hindrance to use it for devices. Whereas, the possibility of precise control of the nanocrystal size and separation by plasma decomposition of SiH4 as described by Y. Kanemitsu, S. Okamoto, M. Otobe, and S. Oda in Phys. Rev. B 55, R7375 (1997) and also by S. Oda and M. Otobe in Mater. Res. Soc. Symp. Proc. 358, 721 (1995) using plasma CVD technique raises the possibility of large numbers of nanocrystal devices with well-defined electrical and optical characteristics. Demand for large area electronics is possible by plasma processing which is an expanding field for microelectronics, optoelectronic and photonic, particularly on the use of capacitively coupled radio-frequency glow discharge technique. Now a days plasma CVD technique is capable of a uniform deposition on glass plates with sizes above 1 m2. There is report about plasma production of nanocrystalline silicon particles and polymorphous silicon thin films for large area electronics devices by P. R. Carbarrocas, A. F. Morral, S. Lebib and Y. Poissnat, Pure Appl. Chem, 74, 359 (2002).
Silane (SiH4) plasmas have been widely studied with respect to the plasma processes (primary and secondary reactions), the plasma/surface interactions, the film growth mechanisms for hydrogenated amorphous silicon (a-Si:H) properties as described by R. A. Street in Hydrogenated Amorphous Silicon, Cambridge University Press, Cambridge (1991). Such amorphous silicon thin films (a-Si:H) are the basis of a large number of devices. Indeed, despite the disordered nature of this semiconductor, its electronic properties are good enough to produce electronic devices such as solar cells, thin-film transistors for active matrix liquid crystal displays, photo and particle detectors, etc. References may be made to Proceedings of the Symposium on Thin Films for Large Area Electronics, E-MRS 2000 published in Thin Solid Films, Vol. 383 (2001). Nevertheless, further improvement of the performances of these devices will require a reduction in the density of defects of the material-related to a reduction of its disorder as well as to enhance its stability. For example, a-Si:H solar cells suffer from the creation of metastable defects upon recombination of photogenerated electrons and holes, which results in a 20 to 30% reduction in the cell efficiency when submitted to prolonged exposure to light. Nanocrystalline silicon could be a answer to the problem faced by hydrogenated amorphous silicon. It has been shown that hydrogenated nanocrystalline silicon (nc-Si:H) films are more stable during light soaking than amorphous silicon as described by J. Meier, R. Fluckiger, H. Keppner, A. Shah in Appl. Phys. Lett. 65, 860 (1994). There is a study by Orpella, C. Voz, J. Puigodollers, D. Dosev, M. Fonrodona, D. Soler, J. Bertomeu, J. M. Asensi. J. Andreu, R. Alcubila in Thin Solid Films, 395, 335 (2001) which shows that thin film transistors (TFT's) made of nc-Si:H show more stability than amorphous silicon TFTs under prolonged times of gate bias stress. However, here it is emphasized nanocrystalline silicon as light emitting material.
Among the various semiconductor materials that have been used to fabricate photonic devices like light emitting diode or lasers, the absence of silicon is striking. The common belief that bulk silicon cannot be a light-emitting material has been severely questioned thus, one naturally wonders why silicon has not been used as a photonic devices. Actually, reasons are associated with the fundamental properties of silicon. Silicon is an indirect band gap material. Light emission is via a phonon-mediated process with low probabilities: Spontaneous recombination lifetimes are in the millisecond range. In standard bulk silicon, competitive nonradiative recombination rates are much higher than the radiative ones, and more of the excited electron-hole pairs recombine nonradiatively. This yields very low internal quantum efficiencies for bulk silicon luminescence. The ability of a material to emit light is usually quantified by the internal quantum efficiency ηint, which is the ratio of the probability that an excited electron-hole pair recombine radiatively to the probability of electron-hole pair recombination. In electronic grade silicon the internal quantum efficiency ηint is ˜10−6. Nonradiative recombination is more efficient than radiative which is the reason why silicon is a poor luminescent material. In addition, fast nonradiative processes such as Auger or free-carrier adsorption prevent population inversion for silicon optical transitions at the high pumping rates needed to achieve optical amplification for laser action. Many strategies have been researched over the years to overcome this limitation, mostly by spatial localization of carriers to decrease the encounter probability with luminescence killer centers. References may be made to L. Pavesi in J. Phys.: Condens. Matter 15, R1169 (2003); L. Pavesi in Materials Today, 18 (2005); P. M. Fauchet in Materials Today, 26 (2005); K. P. Homewood and M. A. Lorenco in Materials Today, 34 (2005).
Hydrogenated amorphous silicon is a single-phase material whereas nanocrystalline silicon (nc-Si:H) can be described as a bi-phasic material consisting of a dispersion of silicon nanocrystals embedded in a matrix of hydrogenated amorphous silicon (a-Si:H). The volume fraction of two phases in nc-Si:H could be varied by selecting the proper preparation conditions. In both phases, a certain amount of hydrogen is dissolved, whose concentration depends as well on the deposition conditions.
Amorphous and nanocrystalline silicon have different electronic and optical properties, and this makes them suitable for different applications. Both types of silicon are made by depositing silicon from plasma of silane that is SiH4 on to a substrate. This plasma deposition technique is straightforward for amorphous silicon but more complicated for nanocrystalline silicon. Normally it is believe that hydrogen from the silane plays an important role in this process, it has been unclear exactly how the two kinds of silicon are related. Recently S. Siraman, S. Agarwal, E. S. Aydil, D. Marodas in Nature 418, 62 (2002) have established that the hydrogen atoms can rearrange the atoms in amorphous silicon into the lattice structure found in nanocrystalline silicon. The researchers proposed that the hydrogen atoms get trapped between loosely bound atoms in amorphous silicon to make a higher energy Si—H—Si configuration. When the hydrogen atoms are later released, they proposed, the silicon atoms would be left in ordered, nanocrystalline array. To test their theory they also used molecular dynamics simulation to show how the deposition process would proceed. They also deposited silicon films from silane plasma in which some of the hydrogen was replaced by deuterium. Structure of nanocrystalline silicon was simulated by R. Biswas and B. C. Pan, NPEL/CD-520-3358, Page 803, NCPV and Solar Program Review Meeting 2003. Biswas's molecular dynamics simulation also shows a mixed-phase material that consists of clusters of nanocrystalline silicon embedded in an amorphous matrix.
Sheets of nanocrystalline silicon are easier and cheaper to make similar to amorphous silicon, so this form has also wide scope for applications where large areas of silicon are needed, including display and solar cells. Moreover, stability is added advantage to nanocrystalline silicon. nc-Si:H has already been demonstrated to be an alternative material to amorphous silicon in photovoltaic applications, with a conversion efficiency of 10.9% in a p-i-n structure type cell configuration as described by A. V. Shah, J. Meier, E. Vallat-Sauvain, U. Kroll, N. Wyrsch, J. Guillet, U. Graf in Thin Solid Films 403, 179 (2002). This is because there is a great advantage of optical properties (low absorption and higher bandgap) in nc-Si:H films as described by Y. He, C. Yin, L. Wang, X. Liu, G. H. Hu, J. Appl. Phys. 75, 797 (1994)] than those of a-Si:H and crystalline silicon (c-Si) in the solar spectrum range. Use of nc-Si:H in photovoltaic is not the only possible technological application, but there is also a great potential for optoelectronics applications. Its structure is considered to be as a dispersion of silicon quantum dots in an amorphous matrix, which turn out to higher band gap material, and easily integral in a silicon chip and fully compatible with the today's microelectronic processes. This is in accordance with the previously reported theoretical studies where it has been mentioned that the quantum confinement effect enlarges the band gap of nanocrystallites as described by Vasiliev, S. Ogut, J. R. Chelikowsky in Phys. Rev. Lett. 86, 1813 (2001), increasing the oscillator strength giving rise to efficient and visible luminescence.
At least in principle optical properties of nc-Si:H might be tuned by controlling the surface rate of the chemical reaction during deposition via a proper selection of the process conditions, with interesting opportunities for the light emitting diode technology as described by S. Binett, M. Acciarri, M. Bollani, L. Fumagalli, H. V. Kanel, S. Pizzine in Thin Solid Films 487, 19 (2005).
There are many methods of producing silicon nano-crystallities, which show photoluminescence, and their efficiency depends upon how the bulk or surface defects from such nano crystallites are removed or passivated. Nano crystalline films grown by plasma enhanced chemical vapour deposition (PECVD) containing silicon, oxygen and nitrogen which is described by M. Ruckschloss et al. in J. Lumin 57, 1 (1993) and by M. V. Wolkin et al. in Phys. Rev. Lett. 82, 197 (1999); Annealed silicon rich silicon oxide films produced by ion implanting silicon atoms into silicon dioxide which is described by K. S. Min, K. V. Shcheglov, C. M. Yang, Harry A. Atwater, M. L. Brongersma and A. Polman in Appl. Phys. Lett. 69, 2033 (1996); Multilayers films prepared by annealing amorphous Si/SiO2 supper lattices which is described by L. Tsybekov K. D. Hirschman, S. P. Duttagupta, M. Zacharias, and P. M. Fauchet, J. P. McCaffrey and D. J. Lockwood in Appl. Phys. Lett. 72, 43 (1998) and porous silicon which is described by R. T. Coolns et al. in Physics Today 50, 24 (January 1997); P. M. Fauchet and Von Behren in J. Phys. Status Solidi B 204, R7 (197), all these nanostructure show photoluminescence. L. Diehl et al. in Appl. Phys. Lett. P4700 (December 2002) have shown electroluminescence from strain compensated Si0.2Ge0.8/Si quantum cascade structures. Radiative transitions is also observed in Er coupled silicon nanoclusters by G. Franzo et. al. in J. Appl. Phys. 81, 2784 (1997). The origin of the luminescence in nanocrystalline silicon may be from two reasons:    (i) confined excition recombination in nanocrystallites of silicon as described by J. Heitmann et al. in Phys. Rev. B 69, 195309 (2004),    (ii) from defect assisted recombination mechanism as described by L. Khriachtchev et al. in J. Appl. Phys. 86, 5601 (1999); L. Khriachtchev, M. Rasanen, S. Novikov and L. Pavesi in Appl. Phys. Lett. 85, 1511 (2004)].In the defect assisted mechanism luminescence is the result of the recombination of carriers trapped at radiative recombination centers that form at the interface between nc-Si and the dielectric or even in the dielectric itself as described by L. Pavesi in Materials Today, 18 (2005); P. M. Fauchet in Materials Today, 26 (2005).
Inventors have observed photoluminescence at room temperature in nc-Si:H films grown by PECVD technique using SiH4 and Ar gaseous discharge. The structure of films may be visualized as nc-Si:H structures embedded in amorphous silicon matrix. In other words nc-Si:H in a-Si:H is as small nanoclusters dispersed in a dielectric matrix. Here a-Si:H can be considered as dielectric because a-Si:H is highly-resistive to nc-Si:H.
Nano structured silicon could provide the alternate to other direct band gap semiconductors, e.g., gallium arsenide and indium phosphide that are good at emitting light but are more expensive than silicon. Porous silicon also known as nanocrystalline silicon emit light but having drawbacks like its brittleness, high porosity and a relatively poor control of the crystallite size, which are major hindrance for its use for devices. Other silicon based light emitting materials like silicon oxide or Si/SiO2 supperlattice structures, SiGe/Si quantum structures, Er coupled silicon nanoclusters are not pure materials. These require doping or complex multilayer structures. Whereas, the possibility of precise control of the nanocrystal silicon size and its separation by plasma decomposition of SiH4 using plasma CVD technique have possibility of making efficient light emitting devices. Moreover, nc-S:H growth by plasma CVD technique is capable of a uniform deposition over large area glass or flexible substrates.
The main objective of the present invention is to provide a process to make photo luminescent nanostructured silicon thin films which obviates the drawbacks of the hitherto known prior art as detailed above.
A novel deposition process has been developed by the inventors to deposit photo luminescent nano structured silicon thin films using plasma enhanced chemical vapour deposition (PECVD) technique which is accomplished through the appropriate selection of various process parameters such as power density, chamber pressure, gas flow rates, temperature etc. Photoluminescence in these nano structured silicon thin films are observed at room temperature. In this process there is no use of doping to obtain photoluminescence. In this process it was found that films deposited under a set of deposition parameters were nanocrystalline in nature and shows photoluminance, which were grown using gaseous mixture of silane (SiH4) diluted in hydrogen (H2), and argon (Ar) in a radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system. Process parameters were varied and optimized to enhance the growth of nanocrystalline phases embedded in amorphous matrix of silicon. These films were characterized for structural properties using X-ray diffraction (XRD), Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Laser Raman techniques. Films were also characterized for estimation of optical bandgap and electrical conductivity.
Among the various semiconductor materials that have been used to fabricate photonic devices like light emitting diode or lasers, the absence of silicon is striking. The common belief that bulk silicon cannot be a light-emitting material has been severely questioned thus, one naturally wonders why silicon has not been used as a photonic devices. Actually, reasons are associated with the fundamental properties of silicon. Silicon is an indirect band gap material. The properties of silicon undergoes significant changes when the crystallite size reaches the range of a few nanometers and start emitting light. Because of their technological applications such nanocrystalline silicon are receiving increasing attention. It appears now that nanocrystalline silicon could provide the alternate to other direct bandgap semiconductors, e.g., gallium arsenide and indium phosphide that are good at emitting light but are more expensive than silicon. Inventors have grown photo luminescent nano structured silicon films on glass and silicon substrates using plasma enhanced chemical vapour deposition technique, which could be useful for making of electroluminance based devices like light emitting diode and LASER.